How do we know that there is more matter than antimatter? Isn’t it possible that some distant objects (maybe even entire galaxies) are made of antimatter?
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If there’s a region of space full of antimatter, then that region of space must share a border with the space we live in full of regular matter.
That border would glow brilliantly and we would see it in our telescopes, but we see no such border.
If it exists way beyond the borders of the visible universe, then we would have no way of knowing, but it would be very odd that it just happened to all be elsewhere.
So we can tell if a given galaxy is made of matter or anti-matter.
Basically, we should be seeing a lot of gamma rays that aren’t explained by anything else if there’s a border between matter and antimatter.
And we simply aren’t seeing that. This means that basically everything we can see is matter.
Two reasons:
1. Antimatter can be detected, and we don’t have any signs showing huge amount of antimatter
2. Occam razor: we can imagine that the universe is significantly dishomogenuos and that what we see locally is wildly different than what happens at the edge of the universe, but it seems unreasonable and it is more efficient to approach in a sort of “innocent until proven guilty” approach.
The photons that come from a matter-antimatter annihilation are very recognizable. If there was a region of space within the observable universe with a significant amount of antimatter then inevitably some would come in contact with matter on the border and we would be able to detect it.
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